The transport of alpha-aminoisobutyric acid (AIB) and alanine (ALA) have been studied during the maturation of cells of the erythroid series. Mature erythrocytes (RBC) of the Long-Evans rat were found to transport both AIB and ALA equally well with and without sodium present in the media. Reticulocytes (RET) were found to transport AIB and ALA to a final distribution ratio of less than 1 if sodium was absent from the media. Final distribution ratios greater than 1 were found if sodium was added to the media. More than 50 percent of both AIB and ALA's transport is sodium dependent. Erythroblastic leukemia cells (EBL), a model immature erythroid cell, showed marked sodium dependence (less than 90 percent) and higher final steady state dispribution ratios than the RBC or RET. In the RBC, all Na-dependent systems for AIB and ALA are lost. In the RET, there are two Na-dependent systems; AIB and ALA, but not n-methyl-AIB (n-m-AIB) and one for n-m-AIB alone. In the EBL, there are at least 3 Na-dependent systems. One for ALA and AIB, but not n-m-AIB, one for AIB, ALA and n-m-AIB, and one for n-m-AIB alone. The younger cells have at least 3 sodium dependent systems; the older cells have 2 which decay to none in the mature RBC.